Manufacture of alkali-metal phosphates



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GLAYTQN 'Fi'E. RUFF, F CLEVELAND, UHIO, ASSIG-NOR Til Tfllf GHASEELLI QHEMIG'AL QLQMPANY, 0F CLEVELAND, Q'HIQ, A GURPQRATK'OIT UTE Q'HIEU. I

MANUFACTURE 01F ALKALi-METAL PHQSPHATES.

No Brewing.

To all whom it may concern.

Be it known that It, SIA'YTON M. Hour, e citizen of the United States, residing at Cleveland, in the county of Cuyuhoga and State of Ohio, have i and useful lmproveincuts in the Manufacture of Alkali-Metal Phosphates, of which the following is a specification.

This invention relates to u. process for the manufacture of alkali metal phosphates.

As is known, the alkali motel phosphates are usually prepared by reacting upon elhalies with crude solutions of phosphoric acid such as are produced by dissolving neturel hosphates in sulfuric acid. Such crude so utions of phosphoric acid contain relatively large amounts of impurities such he: iron, aluminum, calcium, etc, in solution, and if it is attempted to prepare inonoalkali metal phosphates by the addition of the requisite quantity of slkeli, for instance caustic soda or soda ash, thereto the impurities which are soluble in acid remain in solution and contaminate the product. Consequently the preparation of the monoallruli metal phosphates Hi" this way is not satisfactory. Thelirulkali motel phosphates may be prepared in a satisfactory manner spplicetiou illefi'lfuly w, 1924. serial No. 725334..

three alkali metal phosphates or intermedi vented certain new by neutralizing crude solutions of phosphor ic acidwith, for instance, soda ash. in this case the impurities referred to precipitate and may be Separated from the solution prior to'the recovery for instance by crystallization of the (ii-alkali metal salt. The solution however, retains some colored impurities, either organic or colloidal inorganic or both, which unless they are removed or decolorized discolor the di-u1kali metal phosphate product. In the preparution of the tri-alkuli metal phosphate a solution of the oil-alkali motel phosphate is prepared as described, and after the separation of pro-- cipituted impurities is treated with caustic alkali. This process is objectionable on account of the cost of the caustic alkali.

The present invention is designed priate products, that is, mixtures thereof.

The process in accordance with my invention consists generally in subjecting solutions of alkali. metal hos )hates to direct currentelectrolysis in u diaphragm cell.

The electrolysis dissociates the alkali metal phosphates into'allrali .fnetsl and phosphate ions of which the alkali metal ions migrate to the cathode. compartment of the cell where they discharge and unite with water liberating hydrogen and forming caustic alkali, while the phosphate ions migrate to the anode compartment end discharge and react with Water forming phosphoric acid and liberating ouygeu. i ydrogen cud oxygen liberated at the cathode and anode respectively are thus produced as byproducts of the process. The decomposition. of Water with the liberation of: hydrogen and oxygen during the electrolysis results in the com centration oi thesolution and thus assists the subsequent recovery of the roducts in pure form from. the solution. alkali and phosphoric acid formed in the cathode and anode compartments, respective ly, react with the alkali metal phosphates present therein, forming new elueli metal phosphates. I

The electrolysis may he con'inued until all of the alkali metal is transferred to the cathode compartment end all ofthe phosphate to the anode compartment thus producing caustic alkali and phosphoric acid, or the electrolysis may he stopped at any desired intermediate point.

The process is illustrated in the following description of an exemple of the preicrred procedure in accordance with myinvcutiou for the preparetion of the mono and tri alkali metal phosphates from the-di-alkaii metal phos hetes. In this description reference is me e to the sodium phosphates a representative of the alkali metal phosphates. v

A solution of di-sodium phosphate prepared in any suitable manner, for instance by the neutralization of crude phosphoric he caustic acid with soda ash and separation of precipitated impurities and concentratedor diluted if necessary to thedesired concentration, supplied t cell where it isv suhj diaphragm electrolytic ed'to electrolysis. he electrolysis may be carried "out in o variety of ways.

converted to solutions of the mono an For instance, the anode and cathode courwith di-sodium phos hate solution and the operation repeated. his procedure is intermittent. A better procedure is to continuously supply a solution of di-sodium phosphate to t e electrolytic apparatus and continuously withdraw the mono and tri phosphate'solutions. This may bedone by providing a cell of sufiicient length between the inlets and outlets that the di-sodium phosphate solution entering at one end ma be tri sodium phosphates in the anode and cathode compartments, respectively, by the time it reaches the other end. The rate of flow of the electrol to and the current density can of course varied with respect to the length of the cell and its volume ca acity' and the concentration of the electrolyte so as to secure the desired 'conversionof the solution during its flow fromjthe inlets to'the outlets of the cell. The cell in this case should be so constructed, preferably long and narrow and free of abrupt turns and obstructions, as to avoid agitation and mixing of the flowing electrolyte. Still another specific form of ap aratus. which may be referred is a plum ity of relatively small individual electrol ic cells arran ed so that the electrolyte wil flow ,throug series. This arrangement prevents the mixing of electrolyte which has been converted by electrolysis with fresh electrol to and permits a careful regulation of the e ectrolysis so that finished mono and tri sodium phosphates free of any excess of either phosphoric acidor caustic soda may be delivered from the anode and cathode compartments,

respectively. Such a series of units of cells is in effect a single cell.

The alkali metal phosphate solution delivered from the electrolytic cell or cells may be treated in any suitable way, for instance for the recovery of the salts in solid form, by crystallization. In some casesit ma be desirable to produce only the mono-so ium phosphateor the tri-sodium phosphate and this may be done, assumin that the trisodium phosphate is desire by su plying a di-sodium phosphate solution to tie electrolytic apparatus, withdrawing the tri-sodium phosphate solution from the cathode compartment as final product, treating-the mono-sodium phosphate solution delivered from the anode compartment with soda ash them in sufficient to convert it to the disodium phosphate and returning the resulting solution with the (Ii-sodium phosphate supply to the cell. In like manner the tri-sodium phosphate produccd by the electrolysis may be converted by the addition of crude phosphoric acid to the di-sodium phosphate and returned to the cell and the mono sodium phosphate only withdrawn as the final product.

It ivill be apparent from the fore 'oing description that the process is capflile of a great variety of modifications for the production of difi'erentproducts. For instance, by the electrolysis of mono-sodium phosphate di-sodium phosphate and phosphoric acid may be made or the electrolysis of any of the alkali metal phosphates may be con-' tinued to the reduction of caustic soda and phosphoric acid. Further variations of the process include the supplying of different solutions to the respective compartments of the electrolytic cell. But from a commercial oint of view at the present time the most lmportant application of the process is for the conversion of (ii-sodium phosphate to mono-sodium phosphate or tri-sodium phosphate or both.

The controllable factors in the process are the rate of flow and concentration of the electrolyte and the current density of the electrolyzing current. In a given apparatus the length and capacity of the cell are fixed, although in case a plurality ofsmall unit cells connected in series are used the number of units in operation may be varied.

The principal control of the recess therefore depends upon the rate 0 flow of the electrolyte, the current density and the concentratlon of the electrolyte. As to the concentration of the electrolyte this factor may. -be varied practically at will without effect upon the results; that is, the process is operative throughout a very wide range of concentrations. -It is therefore preferred to use a relatively concentrated solution in' order to minimize its bulk and to facilitate the crystallization of the alkali metal phosphates from the electrolyzed solutions. The temperature of the solution may be elevated in order to permit the use of more concentrated solutions, that is, to increase the solubilities of the salts. The electrolyte maybe heated by the passage of electric current, that is by the use of a high current density, or it may be heated by the application of external heat. The current density also may vary within very wide limits. A current density of 7.5 amperes per square foot has been used successfully.

In the production of a particular product,

for instance,, tri-sodium phosphate, variations in the extent of electrolytic action upon the solution may be equalized by discharging the electrolyzed solution into a 

